Millimeter wave signal filters can be useful in the wireless communication art, in both the portable telephone and in location transmitting and receiving equipment. These filters can be used, in a single antenna system, to separate signals into incoming and outgoing directional components having slightly different frequencies, i.e., signals in dual filter frequency duplexer relationship. One type of filter is identified as a transitional evanescent mode/comb-line filter, a filter having a plurality of coupled resonators. The transitional evanescent mode/comb-line filter is used to achieve a sharply tuned response and signal separation desired, for example, in the recited communication service.
In some uses, these millimeter wave signal filters are formed by machining one or two pieces of aluminum and then silver plating the machined surfaces. End loading may be tuned through use of adjustable screws. It is believed that these types of filters have not been used as electronically tunable filters due to the lack of an acceptable high-Q tuning element. These machined filters are also generally limited to frequencies at the X-Band (8-12 GHz) and below due to fabrication tolerances achievable in a machining process.
Other filters have been implemented using micromachining processes. These filters are believed to have all been either enclosed stripline filters or loaded cavity resonators.